Immunization of hamsters with vaccinia virus induced peritoneal macrophages (PM) with functional differences in cytotoxicity (51Cr assay) with time after immunization. Employing "syngeneic" tumor cells BHK and PARA-7 and those cells infected with vaccinia or herpes simplex virus, we showed that early immune PM were cytotoxic for all targets whereas those collected after 7 days were cytotoxic only for virus-infected targets. We propose to use this system to investigate the following areas. (1) The existence of subpopulations of PM arising at dfferent times after immunization which are cytotoxic for selected types of targets (virus-infected vs. uninfected tumor cells). We will use established density gradient centrifugation techniques for this. (2) The concept that transition of normal PM to fully tumoricidal PM is a multisignal activation mechanism will be studied. Virus-infected targets may provide a soluble signal that activates nontumoricidal immune PM to a tumoricidal state. Target cell supernatants will be examined for ability to activate PM and for similarities to known PM stimulants such as interferon. (3) We will examine uninfected and virus-infected targets for differences in quantity or type of binding receptors for PM using cold target cell cpmpetition assays and solubilized target cell membranes. Membranes will be solubilized with nonionic detergent and fractions separated by gel electrophoresis. PM receptors will be identified by testing ability of gel fractions to block or stimulate PM cytotoxicity for different targets. (4) We will examine the possible role of virus-induced membrane fluidity and subsequent clustering of binding receptors on PM cytotoxicity. We will examine the effect of increasing target membrane fluidity by heat and chemicals on PM cytotoxicity. Fluidity of treated cells, including virus infected, will be measured by Con A agglutinability.